A typical office chair includes a seat/back assembly which is coupled to a chair base by a mechanism that allows controlled tilting of the seat/back assembly with respect to the base. The base usually has an upright post which carries a lower, stationary housing part of the tilt mechanism. Pivoted to the stationary housing part is an upper housing part that carries the seat/back assembly. The back may or may not be movable with respect to the seat. A spring extends between the two housing parts of the tilt mechanism and normally maintains the parts in a rest position with respect to one another. When a person sitting on the chair leans back, the upper housing part tilts with respect to the lower housing part and the spring is compressed, cushioning the tilting movement. The spring tension is adjustable to vary the degree of cushioning.
Usually, the spring is a cylindrical coil spring which is mounted on the fixed part of the housing so as to project downwardly below the front center region of the seat. A rod extends back through the spring and has an inner end which is coupled to the upper, tilting part of the housing. The outer end of the rod carries a cup-shaped cap which embraces the outer end portion of the spring and bears against its outer end. When the seat/back assembly is tilted back, the rod draws the cap inwardly, compressing the spring. The cap is coupled to the rod by screw threads so that the tension in the spring can be adjusted by turning the cap, causing it to move along the rod.
While this form of spring tension control works reasonably well, it is somewhat awkward to use. A person seated on the chair must bend forwardly, and then reach down and rearwardly between his or her legs, grasp the cap and turn it in the appropriate direction to increase or decrease spring tension as required.